The viscosity of lubricating oils varies with temperature. In general, oils are identified by a viscosity index which is a function of the oil viscosity at a given lower temperature and a given higher temperature. The given lower temperature and the given higher temperature have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM D2270). Currently, the lower temperature specified in the test is 40.degree. C. and the higher temperature is 100.degree. C. For two engine lubricants with the same kinematic viscosity at 100.degree. C., the one having the lower kinematic viscosity at 40.degree. C. will have the higher viscosity index. The oil with the higher viscosity index undergoes less kinematic viscosity change between the temperatures of 40.degree. C. and 100.degree. C. In general, viscosity index improvers that are added to engine oils increase the viscosity index as well as the kinematic viscosities.
The SAE Standard J300 viscosity classification system does not specify the use of viscosity index to classify multigrade oils. At one time, however, the Standard did require that certain grades meet low-temperature viscosities that were extrapolated from kinematic viscosity measurements taken at higher temperatures, for it was recognized that oils that were exceedingly viscous at low-temperatures caused engine starting difficulties in cold weather. For this reason, multigrade oils which possessed high viscosity index values were favored; these oils gave the lowest low-temperature extrapolated viscosities. Since then, ASTM has developed the cold cranking simulator (CCS), ASTM D5293, (formerly ASTM D2602) a moderately high-shear-rate viscometer which correlates with engine cranking speed at low temperatures, and today cranking viscosity limits, determined by the CCS, are defined in the SAE J300 Standard.
Today, it is also recognized that cranking viscosity is not sufficient to fully estimate a lubricant's low-temperature performance in engines. SAE J300 also requires that a pumping viscosity be determined in a low-shear-rate viscometer called the mini-rotary viscometer (MRV). This instrument can be used to measure viscosity and gel formation, the latter by the measurement of yield stress. In this test, an oil is slowly cooled over a two-day period to a specified temperature before viscosity and yield stress are determined. A yield stress observation constitutes an automatic failure in this test, while pumping viscosity must be below a specified limit to ensure that the oil will not cause an engine to experience a pumping failure during cold weather conditions. The test is sometimes referred to as the TP1-MRV test, ASTM D4684.
Numerous materials are used in the formulation of fully-formulated multigraded engine oils. Besides the basestocks, which may include paraffinic, napthenic, and even synthetically-derived fluids, the polymeric VI improver, and the pour point depressants, there are numerous lubricant additives added which act as antiwear agents, antirust agents, detergents, dispersants, and pour point depressant. These lubricant additives are usually combined in diluent oil and are generally referred to as a dispersant-inhibitor package, or as a "DI" package.
Common practice in the formulation of a multigrade oil is to blend to a target kinematic viscosity and cranking viscosity, which is determined by the specified SAE grade requirements in SAE J300. The DI package and pour point depressant are combined with a VI improver oil concentrate and with one basestock, or two or more basestocks having different viscosity characteristics. For example, for an SAE 10W-30 multigrade, the concentration of the DI package and the pour point depressant might be held constant, but the amounts of HVI 100 neutral and HVI 250 neutral or HVI 300 neutral basestock might be adjusted along with the VI improver until the target viscosities are arrived at.
The pour point depressant's selection normally depends on the type of wax precursors in the lubricant basestocks. However, if the viscosity index improver itself is prone to interact with wax precursors, it may be necessary to add an additional pour point depressant of a different type, or an additional quantity of the pour point depressant used for the basestocks to compensate for that interaction. Otherwise, low-temperature rheology will deteriorate, and TP1-MRV failures will result. The use of additional pour point depressant generally increases the cost of formulating an engine lubricant.
Once a formulation has been arrived at that has the targeted kinematic viscosities and cranking viscosities, the TP1-MRV viscosity is determined. A relatively low pumping viscosity and the absense of yield stress is desirable. The use of a VI improver which contributes little to low-temperature pumping viscosity or yield stress is very desirable in the formulation of multigrade oils; it minimizes the risk of formulating an oil that may cause an engine pumping failure and it provides the oil manufacturer with additional flexibility in the use of other components which contribute to pumping viscosity. To minimize the amount, or number of pour point depressants that must be used, it is advantageous that the polymeric viscosity index improver's composition be designed so that it will not interact with wax precursors.
When multigrade engine lubricants are manufactured, the VI improver is introduced in an oil concentrate. The VI improver is dissolved into basestock, such as HVI 100 neutral, before use. In some instances, the lubricant manufacturer may not have concentrate dissolving facilities, so the oil concentrate must be transported to the user as a concentrate rather than as a solid. It is advantageous to be able to transport a VI improver oil concentrate that is not gelled at room temperature. Gelation makes it difficult or impossible to pour or pump the concentrate into lubricant blending vessels.
The star polymers of the present invention are readily produced by the processes described in Canadian Patent No. 716,645 and U.S. Pat. No. Re. 27,145. However, the star polymers of the present invention have molecular weights and compositions which are not taught by the references and are selected to obtain surprisingly improved low temperature performance as a viscosity index improver.
Viscosity index improvers that are hydrogenated star polymers containing hydrogenated polymeric arms of copolymers of conjugated dienes, including polybutadiene made by the high 1,4-addition of butadiene, was previously described in U.S. Pat. No. 4,116,917. In examples 7-10 of U.S. Pat. No. 4,116,917 anionic polymerization of butadiene and isoprene was used to produce star polymers with hydrogenated poly(butadiene/isoprene) tapered arms wherein the star arms contained hydrogenated polybutadiene blocks that were external to the coupled nucleus. In each of these four examples, butadiene represented 44.3 percent weight of the diene content of the unhydrogenated precurser arms, before coupling with divinylbenzene to produce the unhydrogenated star polymer. Since butadiene initially reacts faster than isoprene when the anionic polymerization is initiated with secondary butyllithium, a polybutadiene block is first formed. As the butadiene concentration is lowered through polymerization, isoprene begins to add to the living polymer so that when the polymerization reaction is complete, the chain is made up of a polybutadiene block, a tapered segment containing both butadiene and isoprene addition product, and a polyisoprene block, to give a living tapered polymer that, when coupled with divinylbenzene, produces a star polymer with the polybutadiene content becoming dominant away from the divinylbenzene-coupled nucleus. Since polymerization conditions favored high-1,4 addition of butadiene, rather than 1,2-addition, vinylic pendant group attachment to the carbon-carbon backbone of the linear chain is minimized, so that, after hydrogenation, the external positions of the arms, remote from the nucleus, resemble polyethylene-like blocks. Then, if there is 90 percent 1,4-addition and 10 percent 1,2-addition of butadiene, after coupling and hydrogenation the polyethylene-like segments will contain 18 ethylene --(CH.sub.2 CH.sub.2)-- segments for every --(CH.sub.2 --CHR)--, where R represents a pendant side-chain ethyl pendant group. Thus, the polymerization method used results in the placement of polyethylene-like segments at the external positions of the hydrogenated arms of the star polymer, remote from the coupled nucleus.
The hydrogenated tapered star polymers of Examples 7-10 were blended into a multigrade engine oil to provide examples 27-30 in U.S. Pat. No. 4,116,917. While dynamic viscosity was measured in the cold cranking simulator at 0.degree. F. by ASTM D2602, such measurements do not correlate with pumping viscosity measurements, and pumping viscosity measurements were not reported.
It is well-established that linear block copolymers such as hydrogenated poly(isoprene-butadiene) block copolymers derived from high-1,4 addition anionic polymerization methods have crystalline hydrogenated polybutadiene blocks, whose melting points can be readily determined by differential scanning calorimetry. Star polymer viscosity index improvers containing crystalline polybutadiene blocks are described in U.S. Pat. No. 5,310,490. In example 7, provided in U.S. Pat. No. 5,310,490, anionic polymerization was initiated first with isoprene and secondly with butadiene to produce a poly(isoprene-butadiene) block copolymer which contained 44 percent by weight of butadiene before coupling with silicon tetrachloride. Such placement is exemplified by a star polymer having the diblock arm structure (hydrogenated polyisoprene-hydrogenated polybutadiene-).sub.4 -Si. In another star polymer example (example 10) the order of addition of monomer was reversed so that the polymerization was first initiated with butadiene followed by isoprene, to produce a poly(butadiene-isoprene) block copolymer which contained 34 percent weight of butadiene before coupling with silicon tetrachloride followed by hydrogenation. That process makes a star polymer having the arm structure (hydrogenated polybutadiene-hydrogenated polyisoprene-).sub.4 -Si.
Polymers 7 and 10 were each dissolved in a basestock containing a commercial fumarate-vinyl acetate flow improver (otherwise known as a pour point depressant) and their low temperature contributions were compared in Table 3 of U.S. Pat. No. 5,310,490.